This invention relates to a composition for inhibiting the corrosion of metals in carbon dioxide containing aqueous systems. More particularly, this invention relates to the use of compositions containing amino thiol or amino disulfide compounds and acidic amino acid polymers as corrosion inhibitors in carbon dioxide containing aqueous systems.
Oil bearing geologic formations generally contain mixtures of crude oil and mineral laden waters, hereinafter referred to as formation waters. Oil wells produce a mixture of crude oil and formation water. As wells age, the natural pressures within the formation decrease, thus leading to decreased production of oil. Wells can be artificially pressurized to force the oil and formation water out of the well, however. In offshore oil platforms, sea water is pumped into the wells to displace the oil from the formation. The mixture of sea water and formation water that results from this process is referred to as xe2x80x9cproduced water.xe2x80x9d In many cases, the oil bearing formations have very low levels of dissolved oxygen (anaerobic). Often, the formation waters are saturated with dissolved carbon dioxide (known as sweet wells) with or without dissolved hydrogen sulfide (sour wells), which both result in an acidic pH environment, wherein the pH can range from about 3 to about 6. The acidic, highly mineral laden waters produces an environment that is highly corrosive to metals, particularly to mild steel, which is the most common material of construction for well pipe lines and equipment, due to cost considerations.
Carbon dioxide induced corrosion differs from oxygen induced corrosion in that iron carbonates and other iron salts are the main corrosion products, as opposed to iron oxide formation (rust) in aerobic systems. Mineral scale formation, caused by temperature and pressure changes in the mineral laden formation and especially in produced waters can either stimulate or inhibit corrosion, depending on the type and physical form of the scale and the pH, temperature and other factors. The expense of cleaning and replacing miles of corrosion and scale damaged pipelines and other equipment on remote oil platforms, and the loss in revenues from diminished oil production can be enormous.
Chemical additives, known as inhibitors, are commonly added to prevent the formation of scale and to inhibit the corrosion process. The control of scale and corrosion in offshore oil production has traditionally required complex mixtures of corrosion and scale inhibiting compounds. Many types of corrosion inhibitors are also physically incompatible with scale inhibitors, however. This results in the need to apply these chemicals as separate treatments.
Many classes of chemicals, of widely varying structures, have been used for the inhibition of metal corrosion. See, for example, reviews on inorganic and organic corrosion inhibitors by Nathan, xe2x80x9cCorrosion Inhibitorsxe2x80x9d Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Edition, Volume 6, John Wiley and Sons, New York pp. 317-346 (1965) and by Trabelli et al. xe2x80x9cMechanism and Phenomenology of Organic Inhibitorsxe2x80x9d Advances in Corrosion Science and Technology, Volume 1, Plenum Press, New York, pp.147-228 (1970). Many classes of corrosion inhibitors useful in oilfield applications are highly toxic and in some cases non-biodegradable. Many corrosion inhibitors interfere with the oil-water separation process, which interference results in relatively larger amounts of crude oil contaminants in the produced water that is discharged into the ocean after separation.
There is a growing concern regarding the environmental impact of scale and corrosion inhibitors, as well as the oil contaminants that are currently being released into sensitive marine ecosystems such as the North Sea through their use in the oil production industry.
Polyaspartates are biodegradable, low toxicity materials with known corrosion inhibiting activity. U.S. Pat. No. 5,607,623 to Benton et al. describes the use of polyaspartates to inhibit ferrous metal corrosion in carbon dioxide containing aqueous systems. Polyaspartates are useful corrosion inhibitors, affording 70 to 85% corrosion inhibition in carbon dioxide containing oilfield brines. In addition, U.S. Pat. Nos. 5,116,513 and 5,152,902 to Koskan et al. disclose the use of polyaspartates to inhibit the formation of mineral scale in aqueous systems. The combined effects of scale and corrosion inhibition in a single additive is a particularly useful feature of polyaspartates.
Amide derivatives of long chain amines have been proposed as environmentally acceptable corrosion inhibitors in oil production applications. See for example Darling et al., xe2x80x9cGreen Chemistry Applied to Corrosion and Scale Inhibitorsxe2x80x9d CORROSION 98, Paper No. 207, National Association of Corrosion Engineers (1998). Unfortunately, such materials can be difficult to formulate and can adversely affect the oil water separation process. Thioglycolic acid (mercaptoacetic acid), which is relatively low in toxicity, is known to be an inhibitor of corrosion. Thioglycolic acid has been used as a corrosion inhibitor in oilfield applications, however it is only partially effective at inhibiting corrosion in a carbon dioxide saturated environment See, for example, U.S. Pat. No. 5,853,619 to Watson et al.
There is a need, therefore, for more environmentally acceptable, biodegradable and low toxicity inhibitors of metal corrosion in carbon dioxide containing aqueous systems where hydrogen sulfide also may be present. Further, there is a need for corrosion inhibitors that are compatible with scale inhibitors and that do not interfere with the oilxe2x80x94water separation process. The present invention satisfies these needs.
Ferrous metal corrosion in carbon dioxide containing aqueous systems can be effectively and efficiently achieved with environmentally acceptable materials. In particular, we have found that amino thiol or amino disulfide compounds of the structure: 
wherein R1, R2, R3, R4, R5, R6, R7 and R8 are, independently of one another, members of the group consisting of hydrogen, carboxyl, C1 to C18 alkyl, and C6 to C10 aryl;
Z is a member of the group consisting of hydrogen and covalent bond;
m and n are, independently of one another, 1, 2 or 3; and
p is zero or 1, with the proviso that p is zero when
Z is hydrogen;
in relatively low concentrations are effective inhibitors of ferrous metal corrosion in mildly acid aqueous systems containing dissolved carbon dioxide. Preferred such compounds are cysteine, cystine, cysteamine, cystamine, and salts thereof.
The environmentally acceptable corrosion inhibitors of the present invention are useful for the inhibition of ferrous metal corrosion in aqueous systems that are corrosive due to the presence of dissolved carbon dioxide found in oil producing formations. In oil production terminology, such aqueous systems are referred to as xe2x80x9csweetxe2x80x9d for carbon dioxide containing formations and xe2x80x9csourxe2x80x9d for systems that contain hydrogen sulfide as well. Sweet and sour conditions are both common problems leading to corrosion in oil producing assets in offshore oil production.
The amino thiol and amino disulfide inhibitor compounds suitable for practicing the methods of the present invention are low in toxicity and provide effective corrosion control at concentrations in the range of about 0.1 ppm to about 1000 ppm. Some of the preferred corrosion inhibitors utilized in the methods of the present invention are naturally-occurring materials such as the common amino acids cysteine and cystine.
In another aspect of the present invention, the aforesaid inhibitor compounds are used together with an acidic amino acid polymer such as a polyaspartate and the like. The resulting corrosion inhibiting compositions have a very low environmental impact when released into ocean waters due to biodegradability and low toxicity. In addition, the resulting combined inhibitor compositions are effective for both corrosion inhibition and scale inhibition in acidic oilfield brines containing dissolved carbon dioxide. Surprisingly, the corrosion inhibiting effect of a component inhibitor formulation that includes an acidic amino acid polymer is superior in performance to either of the individual corrosion inhibiting components alone.